1. Field of the Invention
The present invention is concerned with the adsorption of chiral molecules at surfaces or interfaces, with the arrangement and properties of adsorbed chiral molecules, and with devices and methods based thereupon.
2. Description of the Related Art
The technological and scientific use of chiral molecules is widespread [1]. In contrast, the use of adsorbed chiral molecules at surfaces is, at present, much more restricted and includes applications in heterogenous enantioselective catalysis [see references 2-9, listed below], where the process allows the production of specific enantiomers for commercial use, e.g., in the hydrogenation of -ketoesters. In these applications the role of the adsorbing chiral molecule is to bestow chirality to the chiral surface. There is also only a small volume of fundamental scientific literature [e.g., references 10-20] explaining the observed behavior of adsorbed chiral molecules at non-chiral surfaces and interfaces. This shows that chirality can be introduced to the surface in a number of ways ranging from local chiral adsorption [10, 16, 19] to self-organization in chiral domains [10-12, 14, 15], to local chiral reconstructions [10, 18, 19]. It is known that non-chiral molecules adsorbed on a non-chiral surface can give rise to local chirality within an overall racemic system [e.g., 17, 18, 20]. Therefore the utilization of chiral adsorbing molecules is important in creating complete chirality at the surface, however for certain applications it may be sufficient to work with locally chiral systems.
In all recorded cases known to the inventors where surface chirality is observed, adsorption of an enantiomer and its twin would lead to adsorption where each is aligned symmetrically on either side of a geometric mirror plane [10, 11, 17]. An example is the adsorption of R, R-Tartaric acid and S, S-Tartaric acid on Cu (110) single crystal surface, see reference [11] and also FIG. 1, in which can be seen the orientations of single monolayers of enantiomers of tartaric acid on Cu (110). Molecules of different enantiomers orient themselves symmetrically with respect to a geometric mirror plane of the Cu substrate.
When repeating the experiment on the more commercially relevant Ni (110) surface, it was surprisingly found that the alignment and growth direction of the two R,R-tartaric acid and S,S-tartaric acid enantiomers were perpendicular to each other in directions not related by any geometric mirror plane possessed by the bare surface. Experimental details are provided below. There was no obvious explanation for this observation since all previous work had predicted a geometrically symmetrical arrangement. This new phenomenon which had been discovered could not be explained on purely geometric grounds and can only be understood in terms of a magnetic effect in which the magnetization and spins of the surface influence, and are in turn influenced by, the adsorption of the chiral molecule. Detailed ab initio calculations of the molecule/metal system, which included magnetic effects, have been made to confirm that the adsorption and bonding of a chiral molecule at the surface is, in fact, affected by the inherent magnetization of the surface. In particular the chirality of the molecule dictates which surface spin states are involved in the adsorption and bonding process, an effect that has never been proposed nor observed before. Although the connection between chirality and inherent magnetism has been documented [21-25], e.g., for molecules, metal-molecule complexes, bulk solid state compounds and carbon nanotubes, and has been used to demonstrate enantioselective magnetochiral photochemistry in solution, it has never been observed or proposed for adsorbed molecules at surfaces. This effect is also manifestly different to the reported work on chiral monolayers of polypeptides adsorbed at a surface where very large external magnetic fields were applied subsequent to the adsorption process in order to induce the long, polymeric non-bonding pendant chains to take up different orientations, according to a chirality of the helix, the time that the field is applied and the packing density of the monolayer [26].